Field of Invention
This invention relates to a method for the treatment of water to prevent corrosion and, particularly, to the treatment of municipal water.
Background of the Invention
Municipal water distribution systems have, for countless years, been plagued with corrosion of metal surfaces exposed to the water. The corrosion is particularly acute when a galvanic cell is established with dissimilar metals which are connected by an external conductor allowing current flow.
Presently, the state of the art for corrosion prevention comprises saturating the water with calcium carbonate, using a technique or formula described as Langmuir's Index. Under this approach it is intended that equipment downstream of the treatment plant be subjected to a progressive deposition of calcium carbonate which theoretically passivates the metal surfaces and thus prevents further corrosion. This process is popular with many municipal water treatment facilities because of the low cost. The treatment, however, is dependent upon a number of uncontrolled variables, particularly variables such as: total dissolved solids, alkalinity, calcium carbonate, hardness and temperature of the water. For any given pH of the water, these variables must all be precisely controlled for predictable operation. Because of the large variation and generally uncontrollable nature of these variables, the prior art treatments have not been notably successful. It has been reported by Fair, G. and Guyer, J., Water and Waste Water Engineering, Vol. 2, Wiley) (1968) pp. 30:26 that failures to obtain passivation from calcium carbonate deposition results from inadequate concentrations of calcium and bicarbonate, the presence of chloride and sulfate ions, the stagnation of the water, the presence of carbon dioxide which rapidly dissolves any calcium carbonate deposits, the pH of the water which varies locally to a large extent particularly in an immediate vicinity of a corroded metal, electrical charges at the metal surface and the buffering capacity of the water. Thus, it is not surprising that a truly successful treatment method for inhibition of corrosion of municipal water supplies has not yet been developed.
Prior techniques which have also been practiced include the addition of phosphate corrosion inhibitors to the water. The addition of phosphates is highly unsuitable, particularly to an environmentally conscious public. Additionally phosphates cause difficulty during subsequent treatment of the water in sewage plants and the like. Thus, inhibition of corrosion with phosphates has largely fallen into disfavor and its practice has been sharply curtailed.